Electronic versus steric effects of pyridinophane ligands on Pd(iii) complexes

Several new Pd^II and Pd^III complexes supported by electronically and sterically tuned tetradentate pyridinophane ligands ^MeN4^OMe, ^MeN4, and ^tBuN4 were isolated and fully characterized (^MeN4^OMe: N,N'-dimethyl-2,11-diaza[3,3](2,6)-para-methoxypyridinophane; ^MeN4: N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane; ^tBuN4: N,N'-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane). Cyclic voltammetry studies, UV-vis and EPR spectroscopy, and X-ray crystallography were employed to reveal that the steric properties of the N-substituents of the ^RN4 ligands have a pronounced effect on the electronic properties of the corresponding Pd^III complexes, while the electronic tuning of the ligand pyridyl groups has a surprisingly minimal effect. An explanation for these observations was provided by DFT and TD-DFT calculations which suggest that the electronic properties of the Pd^III complexes are mainly dictated by their frontier molecular orbitals that have major atomic contributions from the Pd center (mainly the Pd d_z atomic orbital) and the axial N atom donors.